[Runtime] Improved content type matching

[czechboy0]

Motivation

The runtime changes for apple/swift-openapi-generator#315.

Modifications

• Introduces a new SPI method Converter.bestContentType that takes a received content type value and from a provided list of other content types, picks the most appropriate one. This actually follows the specification now, by going from most specific (including parameter matching) to least specific (most wildcard-y).
• Deprecates the previously used methods Converter.makeUnexpectedContentTypeError and Converter.isMatchingContentType.

Result

SPI methods that the generated code can use to correctly match content types.

Test Plan

Added unit tests.

[gjcairo]

Looks great overall, just left a few comments with some pretty small nits.